This invention relates to a process in which we use an external catalyst cooler ("ECC") for dehydrogenating and cracking an alkane stream by contacting it with a fluid catalytic cracking ("FCC") catalyst to produce olefins; the olefins are then oligomerized to gasoline in a single zone of a fluid bed crystalline zeolite oligomerization catalyst, the bed operating in the turbulent regime.
Catalyst from a FCC unit is regenerated in a FCC regenerator operating at high temperature due to the high heat release of burning coke. Hot regenerated catalyst (regent catalyst) is conventionally cooled in a catalyst cooler ("catcooler") by generating steam. The catcooler may be either internal or external. In this invention, we cool the regen catalyst in the ECC which also functions as a dehydrogenation reactor to which the alkanes are fed.
Coupling the dehydrogenation of C.sub.2.sup.+ alkanes, and particularly a lower C.sub.2 -C.sub.6 alkane, preferably a mixture of propane (C.sub.3) and butane (C.sub.4) containing a minor amount by weight of olefins and C.sub.5.sup.+ alkanes, in the ECC, with the operation of a FCC regenerator is described in greater detail in our concurrently filed patent application Ser. No. 144,990 the disclosure of which is incorporated by reference thereto as if fully set forth herein. FCC regenerators are designed to be "hot-operated" under pressure, that is, operated at a pressure in the range from about 25 psig to 40 psig, and as high a temperature as is practical from a materials standpoint. The temperature within a FCC regenerator typically ranges from about 538.degree. C. to about 815.degree. C. (1000.degree.-1500.degree. F.) and the ECC operates in our process, in the same general range of pressure and temperature.
Because the dehydrogenation reaction is partly pyrolytic and partly catalytic (effect of the FCC catalyst), the catalyst is referred to as "dehydrogenation catalyst" or "ECC catalyst" when it is in the ECC, and we refer to the catalyst as "regen catalyst" when it is being regenerated. The thermal dehydrogenation of normally liquid hydrocarbons at a temperature in the range from 538.degree. C. to 750.degree. C. (1000.degree.-1382.degree. F.) by pyrolysis in the presence of steam, is disclosed in U.S. Pat. Nos. 3,835,029 and 4,172,816, inter alia, but there is no suggestion that such a reaction may be used as the basis for direct heat exchange, to cool regen catalyst in an ECC for a FCC unit, and provide a mixture of alkenes (or olefins, mainly mono-olefins) and alkanes (paraffins) in the ECC's effluent, as we have done for the first stage of our two-stage process.
The desirability of upgrading lower alkanes to gasoline, distillate and lubes has long been recognized and U.S. Pat. No. 4,542,247 discloses a process for doing so, requiring two oligomerization zones and separation of the effluent from each, to recover the gasoline values.
The oligomerization of lower olefins, alone or in a mixture with alkanes over a ZSM-5 type catalyst having controlled acidity has been disclosed in U.S. Pat. Nos. 3,960,978 and 4,021,502 to Plank et al., and improvements have been disclosed by Garwood et al in U.S. Pat. Nos. 4,150,062; 4,211,640; and 4,227,992, the disclosures of all of which are incorporated by reference thereto, as if fully set forth herein. The oligomerization to gasoline range hydrocarbons ("gasoline" for brevity) is referred to as the Mobil Olefin to Gasoline, or MOG process, and, in the prior art, is preferably conducted over HZSM-5 at moderately elevated pressure and temperature in the ranges from about 6869 kPa (100 psia) to about 3445 kPa (500 psia), and from about 300.degree. C. to about 500.degree. C., respectively. Our MOG reactor also operates in the same temperature range, but preferably at a pressure lower than 689 kPa (100 psia), for example about 275 kPa (40 psia).
The prior art did not recognize that, particularly for the production of gasoline range hydrocarbons from lower olefins, there would be a great economic advantage if the olefins could be obtained at the proper oligomerization temperature, substantially without cost, and could be oligomerized at relatively lower pressure than previously thought desirable. Since, in the real-life operation of a refinery, lower olefins, and especially C.sub.4.sup.=+ (butenes, and higher) are valuable for alkylation, etc., it is only particular economic circumstances which justify their use in a MOG unit. The capability of generating these olefins in the ECC without the inefficiencies of conventional indirect regen catalyst cooling provides an unexpected economic impetus to our two-stage process, at the same time providing a source of olefins for other refinery needs.
Further, in this two-stage process, the first stage will also convert light straight run (C.sub.5 and C.sub.6) alkanes, and C.sub.5.sup.+ paraffinic raffinate, (such as Udex.sup.R raffinate) to olefins because the conversion of all available C.sub.3.sup.+ alkanes proceeds with excellent yields at essentially the same process operating conditions of the ECC.
A still further benefit of "tying" the operation of the MOG reactor to the ECC and the FCC unit is that a portion of the spent catalyst from the regenerator for MOG reactor may be withdrawn and introduced into the FCC cracker, instead of being discarded. In this manner, the activity of the MOG catalyst in the MOG reactor may be maintained at the desired optimum, and the otherwise-discarded catalyst functions as an effective catalytic cracking octane enhancer additive.